1. Field of the Invention
The present invention relates to a phase change memory device, and more particularly, to a phase change memory device that has improved reliability in a read operation.
2. Description of the Related Art
Examples of a nonvolatile memory device that uses a resistance material include a phase change random access memory (PRAM), a resistive random access memory (RRAM), a magnetic RAM (MRAM), and the like. A dynamic RAM (DRAM) device or a flash memory device stores data by using an electric charge, while a nonvolatile memory device that uses a resistance material stores data by using a change in state of a phase change material (PRAM), such as a chalcogenide alloy, a change in resistance of a variable resistive material (RRAM), and a change in resistance of a magnetic tunnel junction (MTJ) thin film due to a magnetization state of a ferromagnetic substance (MRAM).
The following description uses a phase change memory cell as an example. A state of a phase change material is changed to a crystal state or an amorphous state while being cooled down after a heating process. Since the phase change material in the crystal state has low resistance and the phase change material in the amorphous state has high resistance, the crystal state may be defined as set data or 0 data, and the amorphous state may be defined as reset data or 1 data.
A conventional circuit that reads data stored in the phase change memory cell includes a clamping unit that is coupled between a bit line and a sensing node and clamps the bit line to a predetermined bias level in response to a clamping control signal, a read bias supply unit that applies a current for reading a resistance level of the phase change memory cell to the sensing node in response to a read bias control signal, and a sense amplifier that compares a level of the sensing node with a reference level and outputs the comparison result.
The clamping control signal, the read bias control signal, and the like may be affected by various kinds of noises during the read operation of the phase change memory device. This reduces a sensing margin, and thus, it may be impossible for the sense amplifier to accurately differentiate a set state from a reset state. That is, an operation error may occur during the read operation. Read circuits that improve operational reliability are therefore needed.